THE REGULATION OF ORNITHINE DECARBOXYLASE ACTIVITY IN CONTINUOUSLY DIVIDING CELLS AND QUIESCENT CELLS STIMULATED TO PROLIFERATE

Abstract

The objective of this research was to uncover possible regulatory differences in comparable inductions of ornithine decarboxylase (ODC) in two growth states. ODC activity increases 4-5 fold prior to DNA synthesis both in synchronous populations of continuously dividing cells and in quiescent cells stimulated to proliferate. The regulation of this particular enzyme activity in the two conditions is distinct in three ways. First, the addition of 2.0 mM hydroxyurea (HU) will block ODC induction in continuously dividing cells, while having no effect on ODC induction in stimulated quiescent cells. ODC induction in continuously dividing cells is remarkably sensitive to hydroxyurea, whose major effect is in limiting dATP pools. These data also indicate that ODC induction as a cell cycle event occurring previous to DNA synthesis, is not essential for transit of cells from G₁ into S phase. During a HU block, when ODC induction is prevented, cells arrest in early S phase. In addition, after HU is removed, 20% of the cellular DNA is synthesized before ODC activity ever increases. Experiments pursuing the mechanism whereby HU inhibits ODC induction showed that HU added after the induction has no effect on the enzyme activity. Administration of HU one hour previous to the induction prevents it. Therefore, HU is acting to prevent the process of ODC induction rather than simply effecting the enzyme activity. The decrease in ODC induction is not the consequence of a general cell cycle effect since another biochemical marker of the cell cycle (the activity and isozyme forms of adenosine 3', 5'-monophosphate dependent protein kinase) is not inhibited. In addition, general RNA and protein synthesis rates are not altered during an HU block. The inhibition of ODC is not due to a direct effect of HU on the enzyme, a diamine effect or an induction of the ODC antizyme. Hydroxyurea inhibits ribonucleoside diphosphate reductase (RdPR) and chelates ferrous ion. Experiments with a hydroxyurea analog, a less efficient inhibitor of RdPR, is less capable of inhibiting ODC activity. Addition of dithiothreitol resulting in an increased ferrous ion concentration, does not rescue ODC activity. Therefore, the induction of ODC in continuously dividing cells is presumably dependent upon deoxyribonucleoside triphosphates or their metabolites. The second distinct difference in ODC induction is that the expression of ODC in quiescent cells stimulated to proliferate is biphasic as these cells traverse G₁ and enter S phase. Only one peak of activity is apparent in synchronous cycling G₁ cells. The time interval between the first peak of ODC activity and the onset of DNA synthesis is approximately five hours longer in non-dividing cells stimulated to proliferate than in continuously dividing cells. This implies a different role of ODC in the two growth states. The third difference is that the induction of ODC in cells stimulated from quiescence toward DNA synthesis is sensitive to a microtubule inhibitor, colcemid. A microfilament inhibitor, cytochalasin B has less of an effect. In contrast, ODC induction in continuously cycling cells is not altered by colcemid. The biological half-life of ODC, when examined in both growth states was not different. The results presented here suggest that the regulation of an identical enzyme activity intimately connected with proliferative processes is different depending upon the growth state. The induction of ODC is continuously dividing cells occurs closer in time to DNA synthesis, is dependent upon deoxyribonucleoside triphosphate metabolism and independent of a microtubule inhibitor, colcemid. Further, although a temporal correlation between ODC induction and DNA synthesis exists, ODC is not essential for cellular progression into S phase but is required for the completion of DNA synthesis.